The present invention relates to jacketing of steel pipes or tubes with a thermoplastic envelope, particularly an envelope of polyethylene.
German printed patent No. 17 71 764 (see also U.S. Pat. No. 3,616,006) describes a method of wrapping a thermoplastic ribbon around an axially movable and rotating pipe so that the envelope produced is of a helical configuration; an adhesive is interposed between the strip or ribbon and the tube. In particular, a wide nozzle extrudes a flat thermoplastic strip which runs toward the cleaned, heated, rotating, and axially movable pipe to, thereby, become wrapped around that pipe. The adhesive is concurrently extruded to lodge in between the thermoplastic strip and the pipe. The two extruded ribbons overlap and weld thermoplastically along the edges. Further for the state of the art, see U.S. Pat. Nos. 4,178,200 and 3,972,761.
The method described above is a satisfactory one; but it was found that upon using a high-quality polyethylene of high density (e.g., 0.95), one needs very high pressure to operate the wide nozzle, and the throughput will be lower than in the case of a low-density polyethylene. Moreover, for a rather wide nozzle the flow of the material may become irregular, resulting in surface defects so that the desired areal welding of overlapping strip portions is interfered with.
In a different field of art, extrusion of thermoplastic tubes with a circular cross section, it has been observed that the flow field of the extruded material has rotational symmetry. This fact alone is highly beneficial for the quality of the resulting product which is, for reasons of that symmetry superior to the quality obtainable with a wide flat nozzle. One has used this approach by making thermoplastic foils via such a tube, and cutting and flattening it. One could try to make a polyethylene ribbon in that fashion; i.e., cutting longitudinally such a tube, or just flattening it prior to the wrapping process. Cutting and flattening the tube requires a fairly large distance between extrusion head and pipe unless one will introduce undue stretching into the material, particularly at the edges. Moreover, the handling of freshly extruded polyethylene is not desirable because it is prone to stick and will cool rapidly before being completely flat.